Currently, proposals are made for perpendicular magnetic recording as an alternative to horizontal magnetic recording. In horizontal magnetic recording, recording dots have a magnetization direction along the recording plane in the magnetic recording layer. Perpendicular magnetic recording has an advantage over the horizontal magnetic recording in that recording density can be increased easily. In perpendicular magnetic recording, recording dots have a magnetization direction in the thickness direction of the magnetic recording layer. An example of magnetic disk apparatus for perpendicular magnetic recording is described in Japanese Laid-open Patent Publication No. 2003-157507, in which the magnetic disk apparatus includes a bit patterned recording medium. In the bit patterned medium, magnetic regions as recording dots are spaced equidistantly from each other.
A bit patterned medium has a data recording area which is a non-magnetic area scattered with magnetic regions, and a servo-pattern area which is used for disk access control such as magnetic head positioning control and clock signal generation. The servo-pattern area is formed with a large number of belt-like magnetic regions extending substantially radially of the magnetic disk. In the data recording area each magnetic region is given a magnetization direction as a representation of data to be recorded whereas in the servo-pattern area all of the magnetic regions are given the same magnetization direction by a formatting procedure which is performed e.g. during the manufacturing process.
However, the magnetic disk apparatus equipped with the above-described bit patterned medium has a problem in regard to disk access control, that is, the magnetic regions of a large area are susceptible to undesired magnetization reversal due to external disturbances.
Specifically, referring to FIG. 6A, a magnetic region 100 is composed of polycrystalline crystal grains and includes a plurality of magnetic domains 110 through 130, each of which is bordered by a crystal grain boundary and functions as a unit for generation of magnetization directions P1 through P3. The magnetic domains 110 through 130 included in the magnetic region 100 have a strong magnetic exchange coupling force, and the magnetic region 100 is magnetized in one direction.
FIG. 6B illustrates a magnetic region 100′ which is greater in area than the region 100 illustrated in FIG. 6A. Due to the greater area, the magnetic region 100′ includes a larger number of magnetized magnetic domains (in the illustration, five magnetic domains 110′-150′ are depicted). Taking the middle magnetic domain 130′ for example, this domain is influenced by the magnetic fields MF generated by the sandwiching magnetic domains 110′, 120′, 140′ and 150′. As readily understood, when the area of the magnetic region 100′ is greater, the influence of the magnetic fields MF becomes greater, generating a large demagnetizing field DF acting on the middle magnetic domains 130′. In such a case, where the coercive force of the magnetic domain 130′ is seemingly weakened, the magnetization direction P3 may be reversed rather easily by the external magnetic disturbances. This may also hold for the other magnetization directions P1, P2, P4 and P5.
This affects the disk access control servo-pattern area which contains a large number of magnetic regions which have a larger area than magnetic regions in the data recording area. When the disk is new, the magnetic regions have a perfectly uniform magnetization direction, but the magnetization direction is likely to be reversed by external disturbances and other forces in the magnetic regions. Once the reversing of magnetization direction occurs in the servo-pattern area, it becomes no longer possible to make correct magnetic recognition of the magnetic regions in the servo-pattern area, leading to troubles in magnetic head positioning control and clock signal generation, and to inability to perform the disk access control properly.